Summary:
If a shared `IOBuf` is appended to `IOBufQueue` its tail is not considered writable, but we may write into it in the packing logic.

Fix that, and share the implementation between the pointer and value versions.

Reviewed By: luciang

Differential Revision: D31890708

fbshipit-source-id: 2b590cbcab7028af9421d29babb760869833bc65